Cisco issued a patch, but it did not fix the problem. A spokesman said Tuesday that Cisco has its A-Team working on mitigations and a permanent patch. The company plans to issue a security advisory and a detailed mitigation document later this week, he said.

The researchers, who received funding from the Defense Advanced Research Projects Agency (DARPA), analyzed the Unix-like firmware inside Cisco's phones. They found it was possible to eavesdrop on private conversations by tampering with a subset of Cisco's Unified IP phones in the 7900 series running a version of Cisco IP Phone software up to and including 9.3.1-ES10.

Their attack was demonstrated at the Chaos Computer Conference in Hamburg last month.

Once the device is hacked, however, a user of the phone would have no idea the conversation is being monitored, the researchers said. An attacker can also turn on the device's microphone, which would pick up sounds and conversations in the vicinity of the phone.

Cui designed a small wired device, which he calls the "thingp3wn3r," which delivers attack code by hooking into the RJ11 serial port of a Cisco phone. He used a mobile phone to connect to the "thingp3wn3r" over a Bluetooth connection in order to remotely deliver the exploit, according to the video. The researchers say a phone could also be attacked remotely by penetrating an organization's network.

In a security advisory, Cisco describes the vulnerability as a failure to properly validate certain system calls made to the kernel of the device.

"This failure could allow the attacker to overwrite arbitrary portions of user or kernel space memory," which could allow an attacker to take complete control of the device, according to Cisco.

Cui is working on a defense technology called "Software Symbiotes" that is designed to protect embedded systems, such as printers and routers, from code-injection attacks like the one performed against Cisco's phones. The technology would protect Cisco's IP phone, and Cui plans to demonstrate it in the future at a conference.